Ultrasound Technology

What it is ... 

Ultrasonic instruments offer test flexibility in that these instruments can be used for electrical, mechanical and leak inspection. If the goal is to have a truly world-class predictive maintenance programme, the use of multiple technologies is recommended for various inspections. Just as a physician uses multiple tools to take vitals and diagnostics to determine aches, pains and abnormalities, maintenance professionals should take the same approach when it comes to the assets that we are responsible for in our facilities. In addition to using infrared and vibration, ultrasound can be used along with other condition monitoring technologies.

Airborne and structure-borne ultrasound can give the user an ‘image’ of sound, which can be used to analyse, diagnose and confirm mechanical and electrical conditions as well as determine fluid flow issues (for example valve leakage). The concept of ultrasound imaging is to record sounds heard via the ultrasonic instrument. Since these instruments will heterodyne (electronically translate) the sound into the audible range, the recording can be heard simultaneously as it is played back in spectrum analysis software. This will provide the inspector with the sound quality as heard in the field during the inspection along with a visual ‘image’ of the recorded ultrasound. This method will add to the diagnostic process by reducing subjective factors such as sound quality and adding an objective component that includes trending of decibel levels along with sound analysis.

The two types of sensing transducer (airborne and structure-borne) direct the received ultrasounds to a central processing unit, which may be analogue or digital. A central processing unit provides test data through two channels: auditory and visual. The ultrasonic signal is translated into the audible range via a process called heterodyning (see Section 6.5 Measurement techniques). Visual information is viewed on a display panel or meter representing signal strength. In the case of an analogue display this may be a needle indicator meter with some form of calibrated intensity increments or an LED bar graph. Digital instruments display intensity as decibels and provide a reliable indication of ultrasound level changes that can be used to track or trend test points.

Airborne transducers receive ultrasound that is transmitted through the atmosphere. The peak frequency response is usually centred around 40 kHz. These transducers are housed in an assembly that is generically referred to as a ‘scanning module’. A variety of these modules is available to help users adapt for specialised test conditions. For example, in addition to the standard scanning module, there are modules designed to detect airborne ultrasound at a distance, while others are designed for close-up low-level leak detection.

Structure-borne inspections are performed using a metal rod that acts as a waveguide directing the received ultrasound to a transducer. This assembly is referred to as a ‘stethoscope’ or ‘contact’ module. The tip of the waveguide is placed in contact with a test point, usually at a 90º angle, to sense structure-borne ultrasound. In addition to the classical ‘wave guide’ assembly, there are also structure-borne ultrasound sensors that can be magnetically mounted on a test point. These modules contain a transducer embedded in a housing with a magnet on the end. Acting as the aforementioned ‘waveguide’, the magnet conducts the ultrasonic signal directly to the transducer. These sensors are usually attached to the main processing unit via a cable so they can be utilised in hard to reach areas and provide test consistency.

Ultrasonic instruments sense the high-frequency components of friction, ionisation and turbulence. For this reason, these instruments are used for leak detection, mechanical inspection and electrical inspection.